According to the contradiction of theoretical research and experimental result for the thickness variation of ideal plastic bending sheet, starting from the unchanged volume and the plane deformation hypothesis as well as the character that the tangential strain-increment in the strain-increment neutral layer equals to zero, three theoretical resolutions of thinning are presented. The concepts of the three theoretical solutions are correct, the theoretical bases are reasonable, and the deductive methods are strict, which have theoretical value and practical significance for plastic forming theory and stamping process.